The present invention relates to a multilayer printed wiring board and particularly to a multilayer printed wiring board which stacks a plurality of printed wiring boards including an air gap between neighboring printed wiring boards of the multilayer printed wiring board.
An ordinary multilayer printed wiring board is fabricated by stacking a plurality of printed wiring boards inserting a prepreg, which is a sheet of semi-hardened resin, between neighboring printed wiring boards and pressing them under high temperature until they are unified into a hard board. However, a multilayer printed wiring board related to the present invention differs from the above multilayer printed wiring board in that the multilayer printed wiring board of the present invention has an air gap between neighboring printed wiring boards instead of the prepreg. In the multilayer printed wiring board having the air gaps, a plurality of through-hole pads are provided to each printed wiring board and each through-hole pad is commonly placed at the same position on each printed wiring board so that each through-hole pad opposes to another through-hole pad provided to the neighboring printed wiring board when the printed wiring boards are stacked. When the printed wiring boards are integrated, the air gap is left between the printed wiring boards adjacent to each other by inserting spacers into a joined part of the end faces of the through-hole pads provided to the printed wiring boards respectively. The multilayer printed wiring board having the air gaps has been disclosed by U.S. Pat. Nos. 4,368,503 and 4,528,072. According to the U.S. Patents, air gaps were provided for raising up transmission speed of signals each transmitted along a conductor pattern printed on the printed wiring board. Generally, when a signal is transmitted along a conductor pattern and there is a substance between the conductor pattern and another conductor pattern, such as an earth line or an earth plate, which runs near by the conductor pattern for the signal, the transmission speed of the signal is defined that the transmission speed is in inverse proportion to a square root of the dielectric constant of the substance.
In a case that a multilayer printed wiring board is fabricated into a hard board as mentioned before, since resin is usually used as the substance, the transmission speed of the signal becomes inverse proportion of a square root of the dielectric constant, such as 4.5 through 4.9, of the resin. Even though fluorine-contained polymer is used as the resin, the speed is scarcely improved as much as inverse proportion of a square root of its dielectric constant such as 2.3 through 2.9. Consequently, using no substance is most preferable for obtaining the highest speed of the signal. Actually, air is the most preferable substance because its dielectric constant is approximately 1.
FIGS. 1(a) through 1(d) are schematic partially sectional views of the prior art multilayer printed wiring board having the air gaps, and FIGS. 1(a) through 1(d) illustrate the fabrication process of the prior art multilayer printed wiring board. First, through-hole pads 3 and conductor patterns 2 and 2' are formed on a board 1 by etching and plate technique as shown in FIG. 1(a). Particularly, the through-hole pads 3 are formed at the same position to other boards respectively so that each though-pad is opposed to another through-hole pad on the neighboring board when the printed wiring boards are stacked. Second, a spacer is formed on each end face of each through-hole pad by plate technique as shown in FIG. 1(b). Third, a solder layer is formed on each end face of each spacer by plate technique, as shown in FIG. 1(c). The above process is applied to other boards respectively. Then, the printed wiring boards are stacked and integrated to a multilayer printed wiring board by joining spacers with melted solder layers under proper temperature and pressure. FIG. 1(d) partially illustrates the integrated printed wiring boards. In FIG. 1(d), a board 101 is one of boards being adjacent to the board 1, and a plurality of through-hole pads 103 each having a spacer 104 and conductor patterns 102 and 102' are formed on the board 101. The solder layer 5 plated on each spacer 4 and a solder layer plated on each spacer 104 which is not shown in FIG. 1(d), are melted, so that the through-hole pads 3 and 103, the spacers 4 and 104 would be respectively soldered as shown by solder 5' in FIG. 1(d). However, the solder 5' is depicted as an ideal soldering state. Actually, the melted solder flows as shown by solder 105' in FIG. 2. As a result, two problems will occur: one is a problem that a part of the solder 105' is easy to produce or happens to produce a short-circuit between the through-hole pad 103 and the conductor pattern 102 as shown in FIG. 2, and the other is a problem that the strength at the joined part of the spacers 4 and 104 becomes weak because the thickness of the solder around the joined part becomes thin as shown in FIG. 2. The problems have allowed to decrease production reliability of the multilayer printed wiring boards.